The present invention relates in general to amplitude detection circuits and, more particularly, to a differential amplitude detection circuit with DC offset rejection.
In many electronic systems it is often necessary to accurately measure the amplitude of a signal. The amplitude information can be used in many ways. One such way is to regulate a control loop. In regulating control loops, accuracy and common mode rejection are critical, and differential amplitude detection techniques are used. But even with differential techniques, there can be other contributing cources of error. For instance, when circuits are AC coupled and the signal passing between them is unbalanced, a differential offset voltage can be accumulated in the signal. The offset voltage is a result of the unbalanced signal spending more time, on the average, in one level than in the other level.
In data comunications systems a transmission line to is typically coupled to a receiver with an isolation transformer for isolation purposes. In some coding techniques where the data transmission signal spends more time in one state than another, the isolation transformer develops a saturation current that creates a DC offset at the receiver on the secondary side of the isolation transformer. If the data signal is high more than it is low, the DC offset creates a differential signal error. In the past, when offset errors were of concern in system performance, attempts were made to alter the structure of the transmitted code. The transmitted coding scheme was designed such that the time data resided in a high state was matched to the time data resides in the low state. Thus the system transmission bandwith had to be increased to accommodate the coding overhead. Therefore, prior art attempts at amplitude detection circuits lack features for compensation of offset errors.
Therefore, the detector cannot distinguish between the differential data signal and the differential offset signal.
Hence, a need exists to eliminate differential offset from the differential data signal on the received side of a transmission line.